1. Field of the Invention
The present invention relates to a fibrous material for composite materials and fiber-reinforced metal produced therefrom, and also to a process for producing same.
2. Description of the Prior Art
Fiber-reinforced metals (FRM) have recently come into general use as machine parts and structural members. Among others, FRM composed of an aluminum alloy as a matrix and continuous fibers of ceramics or carbon as a reinforcement are known for their outstanding performance. FRM is light in weight, has a high stiffness, and keeps a high strength at high temperatures (i.e., 200.degree. to 400.degree. C.). FRM is produced by, for example, the squeeze casting method. This casting method is suitable for making parts of complex shape such as automotive parts and precision machine parts.
Reinforcements for FRM usually undergo surface treatment because they are difficult to wet by a matrix metal, especially an aluminum alloy or magnesium alloy, and once they do wet, the reinforcements react with the matrix and undergo degradation. The surface treatment is performed by, for example, the CVD and plating methods. According to these methods, the reinforcement fibers are coated with metal or ceramics in the form of a thin uniform film. These methods, however, have some drawbacks. For example, the thin film is liable to peel off due to the differences between the coefficients of thermal expansion for the reinforcement fiber and the matrix. This lessens the effect of surface treatment. If the coating film is made thicker, the reinforcement fibers become rigid and brittle and are susceptible to damage. In addition, a complex apparatus is required for the surface treatment of individual fibers, which adds to production cost.
On the other hand, continuous filament fibers used as reinforcements also have disadvantages. Where the fibers are used for the production of FRM by the squeeze casting method, the fibers are unevenly distributed in the product. This makes it difficult to control the fiber volume ratio (Vf) in FRM, especially in a case where the Vf is small. FRM reinforced with continuous filament fibers alone greatly varies in strength depending on the direction (axis) tested. For example, FRM made by squeeze casting from an aluminum alloy and reinforced with continuous carbon fibers has a strength of 130 kg/mm.sup.2 in the direction parallel to the fiber axis, whereas it has only about several kg/mm.sup.2 in the direction perpendicular to the fiber axis. On the other hand, FRM produced from short fibers alone is isotropic but generally has poor strength.
Heretofore, there has been proposed the combined use of continuous filament fibers or long fibers with short fibers or whiskers as the reinforcement fibers for composite materials. For example, long fibers are used to form the inside part of FRM while short fibers are used to form the outside part of FRM. In another example, a prepeg for FRM is produced by pressure-forming in the presence of long and short fibers mixed together. In the first example, a complex process is required and the resulting FRM is not satisfactory in strength. In the second example, it is difficult to evenly mix long fibers with short fibers. (It may be possible to attach short fibers to the surface of a long fiber bundle by brushing or other means; but it is almost impossible to attach uniformly short fibers to the surface of individual long fibers).
Continuous filament fibers as a reinforcement have a disadvantage in that they are not evenly dispersed in the matrix when FRM are produced by the squeeze casting method. The amount of continuous filament fibers used for reinforcement is 40 to 60%. However, unevenly dispersed continuous filament fibers in such a large proportion come into contact with one another in the matrix, thus reducing the intended strength of the product.
The compatibility of fibers with any given matrix is greatly affected by the composition of the matrix alloy. Therefore, it should be properly selected according to the properties of continuous filament fibers to be used. For example, when an aluminum alloy matrix containing magnesium, silicon, copper, etc. is incorporated with continuous silicon carbide fibers, the magnesium and silicon degrade the fibers, thus forming brittle silicon crystals, and the copper causes the eutectic phases in FRM to grow. This tendency is pronounced in the case of FRM containing a large amount of fibers. Where the reinforcement is alumina fibers, silicon in the alloy degrades the fibers and magnesium and copper cause the eutectic phases in FRM to grow. Where the reinforcement is carbon fibers, magnesium degrades the fibers at high temperatures (although it increases the strength in the transverse direction) and copper and silicon make the eutectic phases in FRM coarser, reducing the strength in the cross direction. For reasons mentioned above, it has been said that a suitable matrix is pure aluminum which does not form precipitates nor degrades reinforcing fibers. FRM based on a matrix of pure aluminum has a low strength in the transverse direction (the direction perpendicular to the lengthwise direction of continuous fibers) because the matrix itself has a low strength.